Phenytoin (Dilantin) Correction for Albumin/Renal Failure

Phenytoin (Dilantin) Correction for Albumin / Renal Failure

Measured Phenytoin

mcg/mL

Albumin

g/dL

Renal Failure

Corrects serum phenytoin level for renal failure and/or hypoalbuminemia.

Why Use

Helps avoid under- or over-dosing in patients with renal failure or low albumin.

When to Use

Patients taking phenytoin who have renal failure or albumin ≤3.2 g/dL (32 g/L).

Formula

Corrected phenytoin = measured phenytoin level/( (adjustment × albumin, g/dL) + 0.1) Adjustment = 0.275; in patients with creatinine clearance <20 mL/min, adjustment = 0.2. Note: no correction is needed for albumin >3.2 g/dL. This calculation is derived from the original Winter-Tozer formula, which used an albumin coefficient of 0.2, and 0.1 for renal failure.

Pearls / Pitfalls

Phenytoin has a narrow therapeutic window, and is highly protein-bound. The protein-bound phenytoin is what is typically measured by the lab. However, unbound phenytoin is the active portion (it crosses the blood-brain barrier). This calculator helps estimate the equivalent active amount of phenytoin based on typical lab values.

Critical Actions

A therapeutic phenytoin level is generally considered to be 10-20 mg/L (40-80 µmol/L). Levels above this may cause nystagmus, ataxia, lethargy, confusion, and coma.

Advice

Adjust phenytoin dosing based on values. If consistently out of range, consider other antiepileptics. Concomitant use of valproic acid and some other drugs displaces phenytoin from plasma proteins and can lead to erratic levels. Whenever possible, a free phenytoin level should be utilized over a corrected level.

More Information

The “Sheiner-Tozer Equation” is the official name of this correction.